Eighty five percent of the population will experience low back pain at some point. Fortunately, the majority of people recover from their back pain with a combination of benign neglect, rest, exercise, medication, physical therapy, and/or chiropractic care. A small percentage of the population will suffer chronic low back pain. The cost of treatment of patients with spinal disorders, plus the patients' lost productivity, is estimated to be 25 to 100 billion dollars annually.
Seven cervical neck, twelve thoracic, and five lumbar (low back) vertebrae form the normal human spine. Intervertebral discs reside between adjacent vertebrae with two exceptions: (1) the articulation between the first two cervical vertebrae does not contain a disc; and (2) a disc lies between the last lumbar vertebra and the sacrum (a portion of the pelvis).
Motion between vertebrae occurs through the disc and two facet joints. The disc lies in the front or anterior portion of the spine. The facet joints lie laterally on either side of the posterior portion of the spine. The osseous-disc combination of the spine coupled with ligaments, tendons and muscles are essential for spinal function. The spine allows movement (flexion, lateral bending, and rotation), supports the body, and protects the spinal cord and nerves.
The discs change with aging. As a person ages, the water content of the disc falls from approximately 85 percent at birth to about 70 percent in the elderly. The ratio of chondroitin sulfate to keratin sulfate decreases with age. The ratio of chondroitin 6 sulfate to chondroitin 4 sulfate increases with age. The distinction between the annulus and the nucleus decreases with age. These changes are known as disc degeneration.
Premature or accelerated disc degeneration is known as degenerative disc disease. A large portion of patients suffering from chronic low back pain are through to have this condition. As the disc degenerates, the nucleus and annulus functions are compromised. The nucleus becomes thinner and less able to handle compression loads. The annulus fibers become redundant as the nucleus shrinks. The redundant annular fibers are less effective in controlling vertebral motion. The disc pathology can result in bulging of the annulus into the spinal cord or nerves, narrowing of the space between the vertebra where the nerves exit, tears of the annulus as abnormal loads are transmitted to the annulus and the annulus is subjected to excessive motion between vertebra, and/pr disc herniation or extrusion of the nucleus through complete annular tears. Disc herniation can also promote arthritis of the facet joints, which in turn may cause back pain.
The problems created by disc degeneration, facet arthritis, and other conditions such as spondylolysis, spondylolisthesis, scoliosis, fracture, tumor, or infection are frequently treated by spinal fusion. Such problems may include pain in the back or legs, nerve injury, risk of future nerve injury, or spinal deformity. The goal of spinal fusion is to successfully “grow” two or more vertebrae together. To achieve this, bone from the patient's body (spine or iliac crest), or from cadavers, is grafted between vertebrae. Alternatively, bone graft substitutes, such as hydroxyl apatite and bone morphogenic protein, may be used. Any autograft, allograft, cadaver graft, bone graft substitutes, or other materials which are placed between the vertebrae to encourage bony fusion will be collectively referenced herein as “graft” or “bone graft”.
The bone graft is placed between the vertebrae in the disc space and/or over the posterior elements of the vertebrae (lamina and transverse processes). The surgeon scrapes the vertebrae to create bleeding. Blood flows into the bone gage. The scraped bone, blood clot (hematoma), and the bone graft simulates a fracture. As the patient heals, the “fracture” causes the vertebrae to be fused and heal together.
Spinal instrumentation, including cages, may be placed onto or into the spine to immobilize the vertebrae that are going to be fused. Immobilization leads to a higher fusion rate and speeds a patient's recovery by eliminating movement. Existing cages are typically hollow metal or carbon fiber devices placed into the disc space. Often, cages have treads, grooves, and teeth or spikes that engage the cerebral endplates. The hollow center of the cage may be filled with a bone graft. The sides of the cages adjacent to the vertebral end plates contain holes to allow bone growth from one vertebra through the cage to the adjacent vertebra. The bone graft acts as a bridge for bone growth. Cages immobilize the vertebrae and maintain the separation between the vertebrae, a function of the formed disc material. Cages are placed into the disc space after excising a portion of the disc annulus and most of the nucleus. One or two cages may be inserted at each level.
Cages may be placed into the disc space from an anterior or a posterior approach to the spine. Cages may be combined with rods, screws, hooks, plates, or other structures. Combining cages with other instrumentation yields a stiffer construct and presumably increases the chance for a successful fusion. If cages are placed from an anterior approach (through the abdomen), the patient must undergo surgery through a second incision over the back (posterior approach) if the surgeon wishes also to insert rods and screws. To avoid two incisions and increased patient morbidity, many surgeons prefer to insert cages from a posterior approach. Rods and screws can then be added through the same incision.